What is Social Planner?

Legal Definition
In welfare economics, a social planner is a decision-maker who attempts to achieve the best result for all parties involved. In neo-classical welfare economics, this means the maximization of a social welfare function. In modern welfare economics, there is a greater emphasis on Pareto optimality, in which no one's economic status can be improved without worsening someone else's. Pareto-optimal solutions are not unique, and according to the Second Fundamental Theorem of Welfare Economics, a social planner can achieve any Pareto-optimal outcome by an appropriate redistribution of wealth by means of competitive market.

In practice, the social planner role is generally played by a government entity. However, real governments have multiple goals in addition to, or instead of the benefit of their people. This problem is studied in public choice economics.
-- Wikipedia